It is known that a conventional electrical steel sheet usable for producing a motor or transformer is made of a silicon-containing electrical steel or a non-silicon-containing electrical steel. Also, it is known that the conventional electrical steel sheet is coated with an electrically insulating layer. This insulating layer is effective for decreasing eddy-current loss in a core and for increasing resistance to corrosion of the core. The term core as used herein means a punched sheet which has been produced by punching the electrical steel sheet. The insulating layer is also effective for increasing the punching property of the sheet and for preventing the sticking of cores made from the sheets to each other during a stress relief annealing process. However, the formation of the insulating layer is expensive.
Recently, in order to greatly reduce the cost in the production of an electrical steel sheet, the tendency has been to utilize a low carbon lamination steel sheet. In this case, usually, a low carbon lamination steel sheet is continuously annealed and, thereafter, temper-rolled at a reduction of 0.2 to 10%, in order to increase the hardness of the sheet and improve the punching property of the sheet. The temper-rolling processes are also effective for accelerating the growth of crystals on the core during the stress relief annealing process. Such temper-rolled electrical steel sheets are sold as semi-processed electrical steel sheets to users. Usually, the electrical steel sheets are subjected by the users to the punching process for producing desired cores and the stress relief annealing process.
In the case where the electrical steel sheets, which are not coated with the insulating layer, are punch-shaped, and the resultant cores are superimposed on each other and annealed in an annealing furnace, the cores frequently stick to each other. This sticking results in reduction of the magnetic property of the cores. When the cores are stuck, it is necessary to peel them from each other. The peeling operation causes the annealing process to have a low efficiency.
In order to eliminate the above-mentiond disadvantages of the conventional electrical steel sheet and the process for producing the same, the following approaches were proposed.
1. The punched cores were coated with a refractory material. However, this process of applying the refractory material onto the surface of the cores had a low efficiency and resulted in a high coat. Also, when such a shaped core was annealed, the resultant core had a poor space factor and undesirable appearance due to the refractory material layer formed on the core surface.
2. When the cores were stuck to each other during the stress relief annealing process, the cores were peeled from each other mechanically by, for example, vibrating or bending the stuck cores. However, the use of the vibrating or bending method resulted in the cost of the stress relief annealing process being high. Also, the vibrating or bending operation sometimes caused the cores to be stressed and, therefore, to have a poor magnetic property.
3. The surfaces of the electrical steel sheets were roughened so as to make the shaped cores easily separable from each other. This method was tried in the U.S.A. and applied to low carbon lamination steel sheets. The roughened surface was formed by using a dull rolling mill having a rough-surfaced roller. These rough surfaced electrical steel sheets could be easily decarbonized during the stress relief annealing. However, this method was disadvantageous in that when the rough-surfaced electrical steel cores were stress relief annealed, the resultant annealed cores had a poor space factor and magnetic property.
4. In the annealing process for eliminating internal strains from the shaped steel cores, the cores were fed, one after the other, into a continuous annealing furnace without superimposing the cores on each other. However, this method resulted in a high cost and, also, the continuous annealing furnace for this process was expensive.
In the field of surface treating the usual cold rolled steel sheet, one technical approach has been developed wherein a surface of a cold rolled steel sheet is coated with a treating solution containing 5 to 150 g/l of chromic acid, 5 to 200 g/l of a silicon oxide sol or aluminum oxide sol and 5 to 100 g/l of at least one metal compound and; the coated sheet is baked at a high temperature of 400.degree. C. or more. Since the treating solution contains a considerable amount of chromic acid which is not permitted to be discharged from the factory, the chromic acid should be removed from the waste liquid of the above-mentioned method before the waste liquid is discharged from the factory. Furthermore, the high temperature baking process undesirably consumes a large amount of energy.
Under the above-mentioned circumstances, it is strongly desirable to provide a new type of anti-sticking electrical steel sheet and a process for producing the same, which are completely free of the above-mentioned disadvantages.